In conventional slurry pumps of the volute type the section of the pump casing surrounding the periphery of the impeller is of changing cross section. The outer peripheral profile is made to approximate a volute shape having a radius of curvature increasing to a maximum at a point where it becomes tangential to the discharge branch.
Not only does the cross sectional area of this volute section of the casing vary but the cross sectional profile also varies around the periphery of the pump. The normal volute type casing therefore has a complex shape which results in a high cost of patterns and moulds for the manufacture of such parts.
Slurry pumps are often fitted with replaceable abrasion resistant liners. The difficulty of manufacturing casings and liners to a true volute shape is further complicated by the necessity for closely matching the outside shape of the liner and the inside shape of the casing. This matter is further complicated by the nature of the abrasion resistant materials which must be used for construction of the liners.
For example, liners for slurry pumps are often constructed in hard cast alloy irons. These materials cannot be easily machined by normal methods and are therefore used in the "as cast" condition with a minimum of machining. Obtaining the required fit between liners and casing is made difficult because of the complex shape of conventional casings.
Another material often used for slurry pump liners is moulded rubber.
When a pump is required to be fitted with rubber liners the construction is complicated in two ways. Firstly, it is important that the rubber liners fit closely into the casing; the requirement for accurate interior shape for the casing and corresponding exterior shape of the liner is more important than for cast metal liners.
Secondly, since the rubber moulds must be constructed from steel or similar material to produce a smooth accurate finish, manufacturing costs are affected greatly by the complexity of the casing shape.
Pump casings are known wherein the casing is of substantially circular shape, or a combination of part of a single circle and part of a conventional volute shape. However, they suffer the disadvantages of poor hydraulic performance due to too great a departure from the ideal volute, and in addition they are still difficult to manufacture.